Field of the Invention
The invention relates to a method and a system for estimating the potential friction between a vehicle tyre and a rolling surface.
Description of the Related Art
Systems for estimating the potential friction are known in the art. See, for example, documents US 2011/0166761, US 2012/0179327, US 2011/0264300, EP 0 444 772, EP 1 510 428, U.S. Pat. No. 6,094,614.
In the present description and claims, “potential friction” is meant to indicate the ordinate of the absolute maximum point in an engaged-friction/kinematic-quantity curve. The kinematic quantity can be the (longitudinal) slip or the drift angle. In the first case, the engaged friction is defined as the ratio between the longitudinal force Fx exchanged in the plane of contact between the tyre and the rolling surface and the vertical load Fz acting on the tyre. In the second case, the engaged friction is defined as the ratio between the lateral force Fy exchanged in the plane of contact between the tyre and the rolling surface and the vertical load Fz acting on the tyre.
In the present description and claims, “engaged-friction/kinematic-quantity curve” is meant to indicate a curve with a predetermined uncertainty band such as to take into account the dispersion of experimental data of engaged friction and kinematic quantity.
In the present description and claims, transitory rolling condition is meant to indicate a substantially rectilinear rolling condition in acceleration or braking; a condition of cornering at constant speed; or a condition of cornering in acceleration or braking, preferably not extreme.
In the present description, “free rolling” condition is meant to indicate a substantially stationary rolling condition, in the substantial absence of longitudinal and/or lateral forces applied to the tyre.
In the present description and claims:                the terms “lateral” and “laterally” are used to indicate quantities measured in a direction substantially parallel to the rotation axis of the tyre;        the terms “radial” and “radially” are used to indicate quantities measured in a direction substantially perpendicular to the rotation axis of the tyre, i.e. in a direction that intersects the rotation axis of the tyre and lies in a plane perpendicular to such a rotation axis;        the terms “longitudinal” and “longitudinally” are used to indicate quantities measured tangentially to the tyre and substantially perpendicular to the lateral direction and to the radial direction (i.e. in the direction of forward motion of the tyre/vehicle).        
With the varying of the conditions of a tyre-rolling surface system, in other words the operating conditions of the tyre (for example vertical load acting on the tyre, inflation pressure of the tyre, speed, wear, temperature, etc.), the characteristics of the tyre itself (structure, tread compound, etc.), and/or the characteristics and conditions of the rolling surface (presence of slippery elements like snow, ice, leaves, roughness, etc.), the relationship between engaged friction and kinematic quantity is described by a different curve and correspondingly there is a different potential friction.
FIG. 1 represents three examples of engaged friction (Fx/Fz)/slip (ε) curves for three different conditions of tyre-rolling surface (granite, cement and asphalt) system.
In practice, the potential friction identifies a limit condition beyond which the adherence conditions of the tyre begin to degrade, up to an asymptotic condition in which, as the slip increases, the engaged friction remains substantially constant and less than the maximum friction (i.e. the potential friction itself).
In each engaged-friction/kinematic-quantity curve it is possible to substantially identify three regions: linear, non-linear and asymptotic (see regions A, B, C, respectively, in FIG. 1).
The linear region represents substantially a “free rolling” or “steady state” condition in which the engaged friction increases substantially linearly as the kinematic quantity increases. In this region the engaged-friction/kinematic-quantity curves relating to different conditions of the tyre-rolling surface system tend to concentrate close to the origin, practically overlapping one another, so as not to be discerned from one another due to the inevitable characteristic uncertainty of the measurements from which the slip and the engaged friction are obtained.
The asymptotic region represents the aforementioned asymptotic condition in which, as the kinematic quantity increases, the engaged friction remains substantially constant.
The non-linear region represents substantially a transitory rolling condition in which the engaged friction increases substantially non-linearly as the kinematic quantity increases. In this region the engaged-friction/kinematic-quantity curves separate from one another.